In 1970, Arthur Ashkin [1] demonstrated how milliwatts of laser radiation can be used to accelerate end even trap micron-sized particles suspended in liquid and gas. Historically, the first laser trap was relying on two counter-propagating laser beams. Later, Ashkin [2] demonstrated that by focusing a single laser beam very tightly, transparent dielectric particles characterized by a refractive index higher than the refractive index of the surrounding medium could be spatially confined in three-dimensions (3D trapping) near the focus of this single laser beam. The term optical tweezers (or laser tweezers) was coined to define this 3D optical trap relying on a highly focused single laser beam.
When a dielectric particle is located in the electromagnetic field of a laser beam, it experiences two types of forces: a gradient force, attracting the particle towards the region of highest electric field intensity, and a scattering force, acting on the particle in the light propagation direction. In an optical tweezers, in order to create a stable axial equilibrium position close to the beam focus, the gradient force has to overcome the scattering force. The ratio of these two forces depends on the degree of focusing of the laser beam, and a stable equilibrium position in 3D can be created provided that the laser beam is focused with a NA exceeding 0.75. Such a tight focusing is commonly achieved by directing the laser beam through an objective lens with high numerical aperture (NA). 3D trapping can already be achieved if the NA of the objective lens exceeds 0.75. However, in order to maximize the trapping performance of the optical tweezers, objectives with NA>1 are usually employed.
Typical examples of particles that can be trapped are transparent micrometer sized dielectric particles (e.g. polystyrene or silica particles), nanometer sized metallic particles, as well as living biological cells [3] and even neutral atoms. The particles are commonly immersed in a fluid medium whose refractive index is lower than that of the particle itself (water very often). For trapping biological particles the wavelength of the trapping light is typically selected in the near infrared range, where the low absorption coefficient of water, cells and cell constituents avoids damaging the trapped biological particles. Recent progress in the area of micro-fluidics has added a new dimension to the development of optical traps. Controlled handling of tiny quantities of liquids e.g. for lab-on-a-chip devices, may prove beneficial in the development of miniaturized bio-chemical reaction chambers. In this context, large arrays of optical traps may allow investigating parallel and simultaneous (bio)chemical reactions on free-floating arrays of (bio)chemical objects—such as cells, cell fragments, nano-containers, or surface-functionalized beads—for drug screening, sorting, recovery of rare primary cells or assessing statistical data on bio-reactions simultaneously taking place in large ensembles of animal cells, bacteria or vesicles. Optical trapping is fully compatible with standard optical diagnosis techniques, such as fluorescence labelling, fluorescence lifetime imaging (FLIM), fluorescence resonant energy transfer (FRET) or Raman spectroscopy.
In this context, trapping in 3D is important for immobilizing biological objects without contact to the surfaces; artifacts often induced by surface immobilization are excluded and sticking of particles is avoided, allowing the particles to be released simply by turning off the trapping laser. Another important advantage of optical tweezers is that particles are trapped at the observing plane of the objective lens. Therefore, as particles are optically trapped, they naturally lie in the ideal position for observation through the microscope. Moreover, the high-NA of the objective lens allows imaging the particles with high spatial resolution, and if the particles or their constituents are labelled with fluorescent markers, the emitted fluorescence light is collected with high efficiency.
By directing multiple laser beams through the same high-NA objective lens, arrays of laser tweezers have readily been demonstrated relying on different techniques, including diffractive elements [6], VCSEL arrays [7] or microlens arrays [11]. Certain optical trapping schemes even allow generating multiple traps that are computer-reconfigurable by laser scanning [10] or spatial light modulators [9]. However, these approaches suffer from certain limitations, the most important one being that the number of objects that can be trapped simultaneously is limited by the field of view of the focusing objective lens. An objective lens characterized by NA=1.25 has a field of view diameter in the order of 200 μm. When trapping living cells having typical sizes of 10-15 μm, this roughly means that no more than 50 cells may be trapped and observed simultaneously. Also, high-NA objective lenses are bulky, expensive, and their extremely short working distance is a restricting factor to the use of optical tweezers in many fields.
A highly non-conventional approach for creating arrays of optical traps would consist in using arrays of micro-optical elements. Provided that each of these micro-optical elements may generate its own optical trap, the number of traps may be increased at will simply by increasing the number of the said micro-optical elements. Another particular advantage of such an approach would be that the micro-optical elements may be mass produced in a parallel fashion using micro-fabrication techniques and also replicated by, e.g. mold casting approaches, to reach extremely low production costs. However, despite the efforts in the micro-optics field for improving the performance of refractive or diffractive micro-lenses, those are still restricted by technological as well as physical limits to relatively low numerical apertures (NA=0.5), meaning that they can not be employed for 3D optical trapping. Although air-immersed two-sided aspheric refractive lenses with NAs as high as 0.7 are commercially available, such a high NA can not be reached with microlenses [8]. For instance, refractive microlenses are commonly manufactured on one side of glass substrate, i.e. they are small plano-convex lenses. Simple calculations show that, for reaching high NAs, the sides of a single-sided aspherical microlens should be very steep relative to the substrate if standard optical glass (n=1.5) is used. Besides the technical issues related to the fabrication of such high aspect-ratio aspherical microlenses, their effective numerical aperture is limited because the steep incidence angles strongly restrict the fraction of light which is effectively refracted at the higher NAs. On the other hand, high-index materials, e.g. silicon, are not employable in the visible and near-infrared ranges due to their poor optical transmission at these wavelengths. Diffractive microlenses (e.g. Fresnel microlenses) also are limited to NAs insufficient for generating optical tweezers, both because of the limited resolution of the manufacturing processes, and because of the rapidly decreasing diffraction efficiency at small grating periods. Finally, graded-index (GRIN) microlens arrays may also be considered, but their NA is typically limited to 0.5, this being related to the technical difficulties in creating very high refractive index gradients within the bulk materials (currently, the best technology seems to be based on silver-ions exchange).
These are essentially the reasons why objective lenses are still conventionally used for optical tweezers. Only a few examples of miniaturized devices capable of generating 3D optical traps without an objective lens have been demonstrated [4, 5]. These systems take advantage of a dual-beam trap [1] configuration (either using two facing optical fibers, or two facing semiconductor lasers) therefore they are relying on a principle different than optical tweezers (which is a single-beam optical trap). However, these approaches are limited to trapping a restricted number of particles; they are unadapted for generating large arrays of optical traps.